This invention relates to the use of optical fibers for transmission of electronic signals which have three levels. The problem leading to the present invention arose from the desire to provide a fiber optic bus extender suitable for use with the military standard data bus MIL-STD-1553, and similar data transmission systems.
Fiber optic lines have major advantages as carriers of optical signals derived from electronic signals on a data bus. Such electronic signals may, for example, be those in aircraft internal time division command/response multiplex data buses utilized in systems integration of aircraft subsystems. It is often desirable or necessary to transmit signals from a data bus to a remote location. Electrical wiring can be used, but the long lines are subject to noise and other transmission limitations.
A fiber optic bus extender will permit bus information to be optically transmitted to, and received from, a remote location, e.g., a location 500 meters or 1,000 meters from the bus. A fiber optic extender is able to protect the transmitted signals from noise interference, in addition to its other advantages over electrical cable connections. A fiber optic extender preferably has two lines, one carrying signals from a bus-connected transmitter, and the other carrying signals to a bus-connected receiver. At the other end of the extender, there may be a monitor, a remote terminal, a bus controller, or another bus.
A fiber optic extender, when used instead of an electrical cable extender, has a significant problem in working with standard bus data systems. This problem results from the fact that bus data having three signal levels must be transmitted by an optical cable which can only carry two signal levels. FIG. 3 of this application is a time diagram which illustrates the difficulties caused by this problem. Solution of the problem is the objective of the present invention.
The difficulties result from the fact that the remotely located unit to which the bus extender carries data must be promptly informed when a given message on the bus has terminated. The transmission system must be cleared promptly for the next message. However, the bus messages have no end-of-message signal which can be carried by a fiber optic extender. Use of an added fiber connection or an added signal frequency to solve the problem would make the system more cumbersome and significantly more expensive. Such systems would also have an increased likelihood of failure.
Over approximately 18 months, various possible solutions, including an unsuccessful system discussed below, were considered and tried for the purpose of avoiding the bus extender difficulties described above, which had the effect of causing loss of bus synchronization. These efforts led to the successful signal transmission system described and claimed in this application.